Implantable medical devices are often used for delivery of a beneficial agent, such as a drug, to an organ or tissue in the body. It is intended that the drug be delivered at a controlled delivery rate over an extended period of time. These devices may deliver agents to a wide variety of bodily systems to provide a wide variety of treatments.
One of the many implantable medical devices that has been used for local delivery of beneficial agents is a vascular stent 100, an example of which is shown in FIG. 1. When coated with a material including a drug, the device is sometimes referred to as a drug-eluting stent (DES.) Vascular stents are typically introduced percutaneously, and transported intraluminally, until positioned at a desired location in the patient. These devices are then expanded either mechanically, such as by the expansion of a mandrel or balloon positioned inside the device, or expand themselves by releasing stored energy upon actuation within the body. Once expanded within the lumen, these devices become encapsulated within the body tissue and remain as a permanent implant.
Known stent designs include monofilament wire coil stents, welded metal cages, and thin-walled metal cylinders with axial slots formed around the circumference. Known construction materials for use in stents include polymers (biodegrade able and biostable,) organic fabrics and biocompatible metals, such as stainless steel, gold, silver, tantalum, titanium, cobalt based alloys, and shape memory alloys, such as Nitinol.
Of the many conditions that may be treated by stent-based local delivery of beneficial agents, one of the most important is restenosis. Restenosis is a major complication that can arise following vascular interventions such as angioplasty and the implantation of stents. Simply defined, restenosis is a wound healing process that reduces the vessel lumen diameter by either extracellular matrix deposition, neointimal hyperplasia, or vascular smooth muscle cell proliferation, and which may ultimately result in renarrowing or even reocclusion of the lumen after intervention. Despite the introduction of improved surgical techniques, devices, and pharmaceutical agents, the overall restenosis rate is still reported in the range of 25% to 50% within six to twelve months after an angioplasty procedure. To treat this condition, additional revascularization procedures are frequently required, thereby increasing trauma and risk to the patient.
Various beneficial agents placed or deposited in or on stents are known. U.S. Pat. No. 5,716,981, for example, discloses a stent that is surface-coated with a composition comprising a polymer carrier and paclitaxel (a well-known compound that is commonly used in the treatment of cancerous tumors.)
Prevention of restenosis has led to formulations intended to lengthen the amount of time over which the medicinal agent is released. The balance between an amount of polymer, solvent, and agent, in a formulation, must be established but is not currently easy to obtain. The consequences of too much agent being released too quickly can be as harmful, in come cases, as not enough agent being released. In addition, the method and structure of the deposition of the formulation on a device must be considered.
An approach to creating a drug eluting stent, or similar device, that results in a safe and effective delivery of therapeutic agent or agents is needed.